Experimental setup and protocol to test the muscle interface. (a) Participants held the handle of a robotic manipulandum whose position was fixed in front of the participant’s chest. Participants flexed and extended the shoulder muscle pair to move the cursor horizontally, and cocontracted the pair to move vertically. (b) Participants first executed point-to-point movements to one of five targets labelled (i) to (v), which were randomly presented in a block design. After the point-to-point reaching task, participants executed via-point movements along four paths, denoted (I)-(IV), for 10 repetitions. The ordering of the via-point targets was fixed. (c) The cursor’s trajectory from a representative participant from the initial and final phases of reaching the point-to-point targets. The length of the trajectory was noticeably shorter in the final phase for some targets. (d) The population mean trajectory length is plotted as a function of the block number (shaded area is one SEM). The trajectories to reach the flexion and extension targets (i) and (v) were the shortest. The length of the trajectory gradually decreased with training as participants learned to manipulate the cursor’s position using the muscle interface. (e) The time-series of the reciprocal activity and the cocontraction from the final phase of the point-to-point reaching task. A linear increase in the flexor and extensor activity was observed. Little RA was observed when reaching the cocontraction target (iii). (f) The time-series of the estimated force from the shoulder flexor and extensor muscles in the point-to-point task. Participants could activate each specific muscle by the correct magnitude to reach the target.